In TW EDM processes, a continuous electrode wire composed of, e.g. brass, and which is commonly as thin as 0.05 to 0.5 mm is axially transported from a supply side to a takeup side continuously to travel through an electrically conductive workpiece while a relative displacement is effected between the traveling wire and the workpiece such that the wire is advanced in the workpiece along a prescribed cutting path transverse to the traveling axis of the wire as material is electroerosively removed from the workpiece by the action of electrical discharges across a machining gap formed between the electrode wire and the workpiece in the presence of a flooding machining fluid, thereby progressively cutting the workpiece along the prescribed cutting path.
The machining fluid is typically distilled water which is deionized through an ion-exchange cartridge to have a specific resistance as high as 10.sup.3 to 10.sup.5 ohm-cm so that it can be effectively dielectric sufficient to cause time-spaced electrical discharges therein. An organic substance which is electrically nonconductive may be added to the deionized water of a relatively low specific resistance to improve the cutting rate. Conversely, a resistivity modifier may be added to the deionized water of a relatively high specific resistance. In all cases, it has been found desirable that the machining fluid with an additive should likewise have a specific resistance in the range described.
In the TW-EDM processes described, there is a constant desire to gain a cutting rate or speed as high as possible. It is desirable that the cutting rate as measured in terms of the rate of advance of the electrode wire be a maximum obtainable for a given level of the machining current. Such cutting rates have heretofore been rather limited.
The present inventor has observed that such limitations limited on the cutting rates obtainable in the prior art are ascribable to the fact that the machining current does not always concentrate at a machining gap of real significance, i.e. between the frontal surface of the advancing electrode wire and that portion of the workpiece which is juxtaposed therewith. In practice, it has been found that the machining current tends to leak through the region of the cutting groove which develops behind the advancing electrode wire and this is due to the presence, in that region, of a portion of the machining liquid externally supplied uncontrolledly to a cutting zone. The machining current tends to leak through the machining liquid of a specific resistance in the range described and due to eroded particles entrapped therein, having an increased conductivity in that region.